FIG. 1 is a block diagram of a Code Division Multiple Access (CDMA) communication system as described in U.S. Pat. No. 6,169,732, and incorporated by reference herein. As described in the '732 patent, the data path between Centralized Base Station Controllers (CBSCs) 111 and 112 retains the Radio-Link Protocol (RLP) framing information. RLP initialization/synchronization is established between each mobile (101 and 102) and its CBSC counterpart as for a normal land to mobile or mobile to land calls. After synchronization is established, each RLP maintains the standard negative acknowledgment (NAK) based control messaging between CBSC and mobile, and data frames are forwarded to the target CBSC's Modified RLP (MRLP) and then to the mobile with RLP framing information.
When an erasure occurs on the up-link data path, the frame is Nak'ed by the source CBSC's corresponding MRLP as in a normal call. Subsequent data frames are forwarded to the target CBSC rather than buffered. The target CBSC forwards these frames to the mobile with the expectation (according to standard RLP implementation) that the mobile will NAK the missing frame and buffer subsequent data frames until the missing frame is retransmitted and received. The down-link MRLP will also detect the missing data frame and record this information to determine which NAKs received from the mobile are to be honored (i.e. due to down-link losses) and which have already been Nak'ed by the peer MRLP application (i.e. due to up-link losses). When a NAK frame is received by the up-link MRLP process, it compares the NAK request with the recorded information to determine if the NAK was expected due to a known missing up-link frame or is a new lost frame caused by a down-link erasure.
FIG. 2 illustrates a typical frame erasure scenario using the MRLP approach. In this case, the missing frame is Nak'ed by source CBSC 111 but frames are still forwarded to target CBSC 112 MRLP for transmission to mobile 102. When mobile 102 detects the missing frame, it will NAK the frame to target CBSC 112, however, target CBSC 112 is aware that this frame had already been Nak'ed by source CBSC 111 and is on its way, so target CBSC 112 will ignore this request. The net result is that the frame erasure on the up-link results in only a single frame data hole as seen by mobile 102, which is equivalent to the performance of RLP for land to mobile and mobile to land calls.
Recently it has been proposed to add dispatch capabilities to the CDMA systems. Unlike the interconnect services provided by today's cellular systems, dispatch services have been traditionally provided by two-way radio systems. Such services allow a user to communicate in ways that are difficult or costly using today's cellular systems. The dispatch group call service, for example, enables a user to communicate with a group of people simultaneously and instantaneously, usually just by depressing a push-to-talk (PTT) button. Using a cellular system, such a call could not occur instantaneously since either telephone numbers would need to be dialed for a three-way call or arrangements would need to be made to setup a conference call.
Likewise, the dispatch individual call service enables a user to communicate with another user quickly and spontaneously. This feature is ideal for two people who are working together but are unable to speak with one another directly such as two people working in concert but in different parts of a building. Where a wireless telephone call is more appropriate for a conversation, short messages between two people as they work are better facilitated by the dispatch individual call service.
Traditionally voice services over cellular systems are not used with NAKing protocols due to concerns with the additional speech delays that are incurred due to retransmissions. However, without retransmissions it is important to maintain a low frame erasure rate over the air so as to minimize audio degradation. By allowing retransmissions the frame erasure rate targets can be increased which can result in considerable capacity improvements in a CDMA system. Thus there is a tradeoff of system delay versus system capacity. It has been deemed, for dispatch services, that it is acceptable to introduce a retransmission mechanism (and thus incur some added delay) to improve the number of dispatch calls supported by the system. However, it is still important to minimize system delays where possible. The use of the MRLP mechanism as described in the '732 patent results in considerably less end to end delay than would be incurred if a standard RLP mechanism was used.
For some scenarios the MRLP approach, as described in '732 may be impractical or can be improved upon. For example consider the scenario where two mobiles are already established in a dispatch call and it is desired to move to a group call by adding a third mobile. The original two mobiles have an established RLP session and are on, say, sequence number 100. The third mobile is added and initiates an RLP session with its serving CBSC. RLP sessions are always initiated at sequence number 0 and currently the CDMA IS-95/IS-2000 specifications do not allow for resetting of an RLP session to a non-zero value. When the third mobile is added and MRLP is applied the sequence number received at the mobile will jump from, say, 1 to 100. This could be interpreted by the third mobile in an incorrect manner. For example the mobile could assume that frames 1 through 99 were lost and will request for their retransmission. Alternatively the third mobile could determine that the RLP protocol needs to be aborted and re-initiated. Similar problems could even occur during the establishment of a private (two users) call, particularly if there is a significant time difference between the two mobiles establishing on their respective traffic channels. Because of this, a need exists for a method and apparatus for a transmission scheme within a communication system that utilizes a modified RLP approach to retransmission and allows multiple users to join a call without the need to retransmit large amounts of data.